docs: def toggle_reciprocal(self):
          """Flip my ``reciprocal_portal`` boolean, and draw (or stop drawing)
          an extra arrow on the appropriate button to indicate the
          fact.
  
          """
          self.screen.boardview.reciprocal_portal = not self.screen.boardview.reciprocal_portal
          if self.screen.boardview.reciprocal_portal:
              assert(self.revarrow is None)
              self.revarrow = ArrowWidget(
                  board=self.screen.boardview.board,
                  origin=self.ids.emptyright,
                  destination=self.ids.emptyleft
              )
              self.ids.portaladdbut.add_widget(self.revarrow)
          else:
              if hasattr(self, 'revarrow'):
                  self.ids.portaladdbut.remove_widget(self.revarrow)
                  self.revarrow = None